This invention relates generally to tubular electric heaters and, more particularly, to a tubular electric heater of flattened cross-sectional configuration which is adapted to be immersed in a fluid to heat the fluid, and which is equipped for reliable sensing of the temperature of the heating element of the electric heater to prevent overheating of the heater.
Immersible tubular electric heaters have a multitude of different uses, as in commercial dishwashers, for example, where one or more heaters are mounted to project generally horizontally from a wall of the dishwasher for heating liquid in the dishwasher. In this application, tubular heaters having a round cross-sectional shape are often used, and energization of the heater is controlled by a heat sensing element which senses the temperature of the heater, the arrangement being such that the element is adapted to deenergize the heater (and thus prevent damage to the heater) in the event the temperature of the heater rises to an excessive level, as may occur when the liquid in the dishwasher drops to a level below the heater.
One such sensing element which has been used is a thermocouple wire or liquid or vapor filled capillary element wrapped around the heater in helical fashion from adjacent one end of the heater to the other. This design presents a problem, however, in that the outer end of the tubular heater tends to droop or sag when heated, so that as the level of the liquid in the dishwasher drops, the portion of the heater adjacent the wall becomes exposed prior to the portion of the heater remote from the wall. Because the sensing element may not sense the rising temperature of the exposed inner portion of the heater, there is a serious risk of overheating and consequent damage to the heater.
Moreover, even if the heater does not droop or sag, only portions of the sensing element are in contact with the upper surface of the heater due to the element's wrapped configuration. As the level of the liquid in the dishwasher drops, the upper portions of the sensing element become exposed prior to the lower portions. This creates a further risk of overheating the heater as the sensing element may not sense the rising temperature of the exposed upper portion of the heater.